The present invention relates to a projection aligner for projecting a mask pattern formed on a mask onto an object to be exposed to transfer the mask pattern to the object.
Projection aligners have been used to form wiring patterns of PCBs (Printed Circuit Boards), for example. A typical projection aligner is provided with a high-power light source such as an ultra-high-pressure mercury-vapor lamp which is provided to emit a light beam toward a mask, and a projecting optical system to project the light beam passed through the mask so that a substrate of the PCB is exposed to the beam passed through the mask and an image of the mask pattern is transferred.
The PCB usually includes through holes for electrically connecting wiring patterns formed on different surfaces thereof. Therefore, the projection aligner have to adjust the positions of the mask and the substrate such that the mask pattern is transferred to the substrate at a correct location relative to the through holes to ensure electrical connection between the wiring patterns.
The substrate of the PCB, however, expands/contracts due to, for example, temperature variation and surface preparation such as mechanical polishing. Such expansion/contraction of the substrate causes displacement of the through holes and, in turn, positional mismatch between the through holes and the mask pattern transferred to the substrate.
In order to prevent the above mentioned positional mismatch, the projection aligner adjusts the size of the mask pattern image formed on the substrate to the that of the substrate by changing the magnification of the projecting optical system.
However, the expansion ratio of the substrate is often not uniform, but varies with directions. Normally, the expansion ratio is larger in the longitudinal direction of the substrate than that in the transversal direction. One of the reason of the above is that the mechanical polishing of the substrate is performed by passing the substrate through a polishing device along its longitudinal direction, which causes the substrate to expand more in the longitudinal direction thereof than in the transversal direction.
Since the expansion ratio of the substrate varies with direction, it has been difficult to adjust the size of the mask pattern image to that of the expanded/contracted substrate to prevent positional mismatch between the through holes and the pattern formed on the substrate.
The present invention is advantageous in that a projection aligner is provided that is capable of appropriately adjusting the size of the image of the mask pattern projected to an object to be exposed to the size of the object even if the object has expanded/contracted with different ratios in different directions.
According to an aspect of the invention, the projection aligner includes a projection optical system that forms the image of the mask pattern onto the object, a expansion ratio determiner that measures lengths of the object in first and second directions and determines first and second expansion ratios based on those lengths. The first and second expansion ratios are defined as expansion ratios of the object in the first and second directions, respectively. The projection aligner further includes a magnification controller that adjusts the magnification of the projection optical system to a value between the first and second expansion ratios.
Since the magnification of the projection optical system is adjusted to a value between the first and second expansion ratios, the size of the image of the mask pattern projected on the object does not differ much from that of the object in both the first and second direction even if the object is expanded/contracted with different ratios in the first and second directions.
In some cases, the object is provided with at least two first marks arranged thereon along the first direction and at least two second marks arranged thereon along the second direction. In such cases, the expansion ratio determiner can determine the length in the first direction based on a distance between the first marks and the length in the second direction based on a distance between the second marks.
The expansion ratio determiner may include a camera arranged to capture image of the object, and an image processor adapted to determine the lengths of the object in the first and second directions from the image captured by the camera. In this case, the image processor may utilize the marks mentioned above for determining the lengths of the object. Alternatively, the image processor may utilize alignment marks of the object for determining the lengths of the object.
Optionally, the first and second directions may be respectively selected to be parallel to longitudinal and transversal directions of the object, especially when the object is a substrate of a printed wiring board since the expansion ratio of such kind of object often differs for a relatively large amount between the longitudinal and transversal direction thereof.
According to another aspect of the invention, the projection aligner includes, a projection optical system that forms the image of the mask pattern onto the object, a length determiner that measures lengths of the mask and the object in first and second directions, and a magnification controller that adjusts the magnification of the projection optical system to a value between first and second size ratios which are ratios of the length of the object to the length of the mask in the first and second directions, respectively.
It is well known that the mask also expands due to temperature variation, for example. The projection aligner configures as above, however, can determine the size ratio between the mask and the object accurately even if the mask has expanded since it also measures the size of the mask. As a result, the projection aligner can form the image of the mask pattern on the object in an appropriate size irrespectively of the expansion of the mask.
Optionally, the length determiner may include first and second cameras arranged to capture images of the mask and the object, respectively, and an image processor adapted to determine the lengths of the mask and the object in the first and second directions from the images captured by the first and second cameras.
Further optionally, the image processor may determine the lengths based on alignment marks formed to each of the mask and the object at the vicinity of each corner thereof.
According to another aspect of the invention, there is provided a projection aligner for transferring an image of a mask pattern of a mask which includes a plurality of local mask areas onto an object which includes a plurality of local object areas. Each local mask area includes a portion of the mask pattern that is to be transferred onto different one of the plurality of local object areas. The projection aligner includes a projection optical system that forms the image of the mask pattern onto the object, and a length determiner that measures lengths of each of the local mask areas and the local object areas in both first and second directions. The projection aligner further includes a size ratio determiner that determines first and second size ratios, which are ratios of the length of the local object area to the length of the local mask area in the first and second directions, respectively, for each of the object local areas. Further, the projection aligner includes a magnification controller that adjusts the magnification of the projection optical system to a value between first and second mean size ratios. The first means size ratios is a mean value of the first size ratios, and, the second means size ratios is a mean value of the second size ratios.
In the projection aligner arranged as above, the size ratios of the object to the mask is determine by taking into account local variations in the expansion of the object. Accordingly, the projection aligner can adjust the size of the mask pattern image so that the mask pattern image is formed on correct location on the object irrespective the local variation of the expansion of the object.
Optionally, The projection aligner may include a plurality of first cameras each arranged to capture images of different one of the local mask areas, a plurality of second cameras each arranged to capture images of different one of the local object areas, and an image processor adapted to determine the lengths of each of the local mask areas and the local object area in both first and second directions from the images captured by the first cameras and the second cameras.
Further optionally, each of the local mask areas and the local object areas may be defined as a rectangular area and has an alignment mark in the vicinity of each corner thereof, and the image processor may determine the lengths based on the distance between the marks.